This invention relates bonded parts to engines and transmissions. In a preferred embodiment it relates to valve cover assemblies for internal combustion engines herein the valve cover is bonded to the cylinder head by an adhesive. The invention also relates to processes for bonding valve covers to cylinder heads of internal combustion engines.
Automobile and truck engine and transmission systems have many parts which are assembled. Because many of these connections connect systems which involve the flow of fluids wherein contamination of the fluids can cause damage to the engine, it is important to both attach the parts and seal the joint between the parts. The parts are typically attached using mechanical fixturing means and sealed using gaskets, some of which are cure in place compositions. A representative system is a valve train component system. Automobile and truck engines typically have their valve train components covered with covers designed to protect the valves and internal components from a variety of external contaminants and to contain engine oil and combustion gases within the engine for proper disposal. These covers are variously referred to as valve covers, rocker covers, cam covers and the like. Traditionally these valve covers have been made from metal materials such as steel, magnesium, or aluminum. Recently, in order to reduce cost and weight, valve covers have been made from thermoset plastic materials or thermoplastic materials which can withstand the high underhood temperature environment, see for example, U.S. Pat. No. 5,492,086; U.S. Pat. No. 5,375,569; U.S. Pat. No. 5,746,168 and U.S. Pat. No. 5,363,759, incorporated herein by reference. Typically the valve covers are attached to the engine by bolting the valve cover to the cylinder head. In order to seal the internal engine components from the outside environment and contain engine oil and combustion gases a gasket is disposed between the valve cover and the cylinder head.
It is well known that some plastic materials creep when exposed to hot temperatures and pressure. This causes concern with the use of thermoplastic materials as valve covers especially, when the thermoplastic based valve covers are tightly bolted in place. Typically, the mechanical fasteners used compress the valve cover and gasket can contribute to this creep. U.S. Pat. No. 5,365,901 incorporated herein by reference discloses the use of a low creep material as a flange that is bolted to the cylinder head wherein the valve cover is mechanically or chemically locked to the flange.
The problem with traditional valve covers is that assembly of a valve cover to an engine is labor intensive and requires costly mechanical fixing devices, such as bolts and threaded bolt holes. The need to use threaded bolt holes also requires significant machining and cost. In addition, the traditional means of assembly requires the use of a gasket to seal between the valve cover and the cylinder head. This too requires significant labor. Further, gaskets tend to creep and crack with age resulting in leaks which impact engine performance and require costly repairs.
Jones et al, U.S. Patent Application 2002-0112684 (incorporated herein by reference) discloses a valve cover adhesively bonded to an engine head wherein the adhesive has sufficient strength to hold the valve cover in place during normal operating conditions, see paragraph 0007. Adhesives disclosed as preferable include a high temperature epoxy resin, a polyimide, a hybrid polyimide/epoxy resin adhesive, a silicone, a fluorosilicone, an alkylborane initiated acrylic adhesive system, or an epoxy novolac/nitrile rubber adhesive, see paragraph 0026.
A problem in bonding a valve cover to an engine head is selecting an adhesive which meets the stringent requirements necessary for use under the operating conditions of a vehicle. The adhesive must maintain strength sufficient to hold the valve cover in place over the life of a vehicle when exposed to hydrocarbons at elevated temperatures of upwards from 150° C. for extended periods of time. Additionally, the adhesive must have sufficient ductility to maintain a seal at low temperatures at which the vehicle may be exposed in northern winters. After assembly, the engine is exposed to a low pressure test to insure it is properly sealed. One location which needs to be sealed is the junction between the valve cover and the engine head. Thus the adhesive needs to form a sufficient seal at engine assembly to seal against leaks during the low pressure testing and to hold the valve cover in place during the testing.
What is needed is a system of bonding parts to an engine or a transmission wherein the system is capable of handling the temperatures, pressures and exposures to a variety of chemicals as found in automotive engine and transmission systems. What is needed is a valve cover bonded to an engine head with an adhesive which has good high temperature strength properties, good hydrocarbon resistance at high temperatures and good ductility to seal the juncture between the valve cover and the engine head at a wide range of temperatures. Additionally, an adhesive which maintains its properties over the life expectancy of the vehicle is needed and which has sufficient strength and ductility to hold engine and transmission parts in place and seal the joints between the parts and the engine or transmission. An adhesive which also has sufficient strength shortly after assembly of the valve cover to the engine head to provide a seal during the low pressure leak test is needed.